The present invention relates to a method for inkjet printing using an ink comprised of an acicular pigment. The present invention also relates to an ink formulation(s) comprised of an acicular pigment for use in printing images with an inkjet printer, and, more specifically, to an ink cartridge and an inkjet printing system.
Inkjet printing has gained wide acceptance in the printing industry because it is a relatively inexpensive form of printing and yet it produces high quality printed images. However, inkjet printing is complex to implement in practice. Although there are a broad range of techniques describing the non-contact printing process of propelling droplets from an inkjet printing device onto a recording medium, there are basically two classes of inkjet printing technology. They are continuous inkjet printing (CIJ) and drop on demand printing (DOD). For example, U.S. Pat. No. 5,463,416 discloses a method of operating a drop on demand inkjet printer.
In common with all drop on demand printing techniques, ink is drawn from an ink reservoir into a capillary channel in a print head. Within the channel, an energy pulse disrupts a portion of the ink into a droplet that is expelled onto a recording medium, such as paper. The energy pulse may be provided by a piezo-electric element, which vibrates to produce the pulse. Alternatively, the external stimulus may be provided by a thermal element, which heats up the ink and forms a bubble that creates a pressure wave, forcing an ink droplet out of the channel. The distribution of the ink droplets are controlled to form the desired image. Also, the droplet volume and the image resolution, in droplets per unit area, are controlled by the inkjet printer system.
There are many different types of drop on demand printing techniques and often each technique has a specific commercial implementation that affects the requirements and properties of the ink. Typically, the inks utilized by inkjet printer systems are usually limited by the viscosity specifications of the print head. For example, a print head may have a designated 8 centipoise (cPs) limit for ink viscosity. If higher viscosity inks are used, then the print head will give poor print quality as drop velocity is lowered and drop ejection becomes more erratic. If a sufficiently high viscosity ink is used, then no drops will be generated as there is insufficient energy generated in the firing process to eject droplets. Therefore, it is important to control the properties, particularly the viscosity and rheological properties of the ink during this process.
Thus, a key physical property to control in formulating inks suitable for drop on demand inkjet printing is viscosity. In general, the requirement is to formulate inks with low viscosity in order to enable the jet formation process to occur. Above a certain viscosity, it is not possible to create a jet or droplets reliably or at all. Typically, the viscosity of an ink is measured under low shear conditions relative to the high shear conditions experienced during the droplet ejection process. In generally available inks, therefore, the designated viscosity limit of the print head becomes the limit for the low shear viscosity of the ink.
Additionally, during the inkjet process, ink is subjected to shear as the droplet is formed. In general, inks that undergo shear thinning during the droplet formation process produce erratic and poor inkjet inks due to the changing viscosity properties of the inks. Thus, in creating stable and reliable droplets, it is critical that the ink fluid is in equilibrium during the changing conditions of producing the droplet. Traditionally, small sub-micron spherical particles have been dispersed within a liquid to achieve reliable performance and the volume fraction determines the viscosity of the fluid.
To form more bright and distinct images, it is often desirable to add pigment particles to the ink, thereby increasing the pigment concentration. The problem with adding pigment particles to the ink, however, is that this typically increases the viscosity of the ink. With the increased viscosity of the ink, the ink""s resistance to flow is increased, requiring a greater amount of energy to expel the droplet. The increased viscosity of a preferred ink, therefore, may exceed the capacity of the print head. When the ink viscosity exceeds specifications of the print head, there is not enough energy generated within the print head to reliably expel the droplet. Thus, poor print quality and printing failure are the result. Thus, viscosity is a key restriction for ink formulators and significantly limits the solids loading of colorant or other material that can be incorporated into an ink formulation. As a result, this limits the number of applications in which inkjet can be used commercially.
As mentioned above, increased pigment concentration is generally desirable in all types of inkjet printing. One specific type of inkjet printing that would benefit from increased pigment concentration is MICR printing. One way to automatically read printed characters is to form the characters with inks that can be magnetized and read by magnetic ink character. recognition (MICR) devices. For example, magnetic ink characters passing through a MICR reader are first energized with a magnetic field and then the magnetic signal from each character is sensed to determine the character. The ability of a MICR reader to recognize a magnetic ink character is a function of the quality of the form of the character and the ability of the magnetic ink to sustain a magnetic field once it has been magnetized, as measured by the ink""s magnetic properties, remnance and coercivity. Maximizing the amount of magnetic particles added to the ink enhances the magnetic ink""s magnetic field strength, and thus, increases the readability of the character.
Current methods of MICR printing produce characters that can be reliably read, however, these methods are relatively sophisticated and expensive. Typically, MICR printing is currently performed using offset presses, laser printers, and impact printers. The offset press process generally involves large, expensive, complicated machinery and magnetic inks/pastes that may be difficult to handle. This type of machinery is used mainly by businesses whose main concern is printing large volumes of materials, as opposed to businesses where MICR printing is a secondary or internal concern. Similarly, MICR printing with laser printers is relatively expensive as the laser printer itself is very costly. Additionally, using magnetic toners is expensive as printing with laser toner cartridges is already one of the more costly methods of printing, while the specialized magnetic toners add even more cost. Impact printers with magnetic printer ribbons are also costly and are not as efficient and reliable as other methods because they rely on mechanical parts to perform the printing. Thus, a low cost, unsophisticated, yet practical and reliable way to perform MICR printing is desired.
Inkjet technology, in general, is attractive because it is relatively inexpensive compared with offset printing and laser printing. Inkjet technology, however, has never been commercially successful for MICR printing as far as can be determined. One of the basic problems is that in order to form a character that can hold the required magnetic field, a large mass of magnetic particles is required in the ink solution. Additionally, the increased mass of magnetic particles produces a higher viscosity solution. The larger mass of particles and resulting higher viscosity solution make it difficult for inkjet printers to form and expel droplets.
Further, once an appropriate combination of magnetic particles, ink viscosity, and inkjet printer head are found, then it is difficult to keep the magnetic particles from falling out of suspension. And when the magnetic particles do happen to fall out of suspension, it is difficult to redisperse the particles because they tend to aggregate or flocculate (clump together). Thus, printing MICR characters with inkjet technology printers has not yet proven to be commercially feasible.
Another problem with magnetic ink printing, especially for inkjet printers, is that it is difficult to maintain a homogeneous mixture of the liquid magnetic ink suspension, as the heavy magnetic particles tend to settle. A similar problem with ink having large and heavy pigment particles has led to the development of devices to agitate the ink wells storing the ink to be printed. These separate agitators, however, require additional parts that increase the expense, and reduce the reliability, of the device.
Further, a thicker or more viscous magnetic ink solution may be utilized to reduce the amount of settling. This produces printing problems, however, especially with inkjet printers. Typically, traditional drop on demand inks have a viscosity in the range of 1.5 to 12 centipoise (cPs) for optimum operation. A centipoise is 0.01 poise, where poise is a unit of dynamic viscosity of a fluid in which there is a tangential force of 1 dyne per square centimeter resisting the flow of two parallel fluid layers past each other when their differential velocity is 1 centimeter per second per centimeter of separation. This limited range is often a factor that restricts the concentration of pigment particles that can be used in developing inks with useful properties.
Thus, there is a need to produce an ink that can be reliably utilized in an economical inkjet printer for printing images.
It is an object of the present invention to provide a method of using an ink comprised of an acicular pigment and a solvent in an inkjet printer to print on a recording medium.
One embodiment of the present invention is a method of using an ink comprised of an acicular pigment and a solvent in an inkjet printer to print on a recording medium. The method involves obtaining an ink which is comprised of an acicular pigment and a solvent and which has a first viscosity, filling the ink reservoir of the inkjet printer with the ink, flowing the ink from the ink reservoir to the print head of the inkjet printer, and jetting the ink from the print head and onto the recording medium. Upon exiting the print head, the ink has a second viscosity. In another embodiment of the present invention, the acicular pigment may be a surface modified acicular pigment. In another embodiment of the present invention, the ink further comprises a dispersant, and in yet another embodiment the ink may further comprise a polymer.
It is another object of the present invention to produce an ink formulation(s) that can be reliably utilized in an economical inkjet printer for printing images. It is yet another object of the present invention to disperse particles with solids loading much greater than is typically used and yet maintain reliable dispersion properties and inkjet performance by using acicular particles with a given aspect ratio. Therefore, in accordance with the present invention, inks have been formulated with very high solids loading using acicular particles as opposed to spherical particles. The viscosity of the inks of the present invention, measured under very low rates of shear are much higher than traditional inkjet inks and would typically be considered to one skilled in the art as too high for reliable inkjet printing.
In one embodiment of the present invention, an ink formulation comprises an alcohol, an acicular pigment, and a transition metal complex. In yet another embodiment of the present invention, an ink formulation comprises diacetone alcohol, iron oxide, and di(dioctyl)pyrophosphate oxoethylene titanate.
It is another object of the present invention to provide an ink cartridge for use in an inkjet printing system.
In another embodiment of the present invention, an inkjet cartridge comprises a) an ink comprised of an acicular pigment and a solvent, and b) an ink reservoir. In another embodiment of the present invention, the acicular pigment is surface modified. In another embodiment of the present invention, the ink further comprises a dispersant. In yet another embodiment of the present invention, the ink further comprises a polymer.
It is a further object of the present invention to provide an inkjet printing system.
In one embodiment of the present invention, the inkjet printing system is comprised of a) an inkjet print head, b) an ink reservoir, and c) an ink comprised of an acicular pigment and a solvent. In another embodiment of the present invention, the acicular pigment is surface modified. In another embodiment of the present invention, the ink further comprises a dispersant. In yet another embodiment of the present invention, the ink further comprises a polymer.